Electrorefining metallic titanium



June 7, 1960 M. J. RAND 2,939,823

ELECTROREFINING METALLIC TITANIUM Filed Sept. 26, 1957 FiG.|

FIG.2

INVENTOR Myron J. Rand ATTORNEYS the fused salt melt.

2,939,823 ELECTROREFININ G METALLIC TITANIUM lVIyron J. Rand, Palmefton, Pa., assignor to The New Jersey Zinc Company, New York, N.Y., a corporation of New Jersey 1 Filed Sept. 26, 1957, Ser. No. 686,466

5 Claims. (Cl. 204-64) amounts of certain interstitial impurities, however; so

embrittles and hardens titanium that it cannot be easiy H worked in the cold, and as a result advantage cannot be taken of the many esteemed properties of this metal. The

most common impurities which cause the damaging embrittlement of metallic titaniumare nitrogen and oxygen.

These elements have an opportunity to enter or com- :bine with titanium metal during the production of the initial metal sponge, during the subsequent recovery and consolidation of the metal sponge into an ingot suitable .for fabrication into useful end products, and during the actual fabrication of these useful end products. Because 1 the production of a useful titanium metal end product also produces a relatively large amount of titanium scrap contaminated with the aforementioned embrittling impurities, there exists a need for a process for purifying or refining this impure titanium sponge and titanium scrap metal.

/ One of the most promising of the processes forpurify- .ing impure titanium metal involves the electrorefining of the impure metal by fused salt electrolysis. In this process anelectrolyzing current is passed betweenthe C impure titanium as the anode and an inert cathode both of which are in contact with a fused halide salt bath containing a lower valent titanium halide. The impure titanium metal at the anode dissolves in the bath and is rde'posited at the cell cathode in the form of substantially pure titanium metal. For best results the cell voltage is maintained below that at which there is evolution of free chlorine or other halogen gas at the anode, and consequently there is no necessity for the internal and external barriers commonly employed in electrowinning cells to separate the anode products from the cathode products.

The electrorefining of titanium under the foregoing .conditions proceeds smoothly and efficiently with the deposition of substantially pure titanium at the cathode.

However, the process often displays rather poor current cfiiciency after it has been in operation for a relatively ---short period of time. I have investigated the cause of the inefliciency of this electrorefining process and have found that one source of such inefliciency is the formation of a thin film or skin of metallic titanium on the surface of The reason for the formation of this skin of metallic titanium on the surface of the melt is not known.. However, once this metallic skin or film forms on the surface of the melt, it offers the path of least resistance for the electrolyzing current between the anode and the cathode. As a result the metallic skin conducts current that accomplishes no electrochemical change so t-hat the process becomes inefficient and wasteful.

I have now found that I can prevent the formation of the aforementioned metallic titanium film on the surface of the'bathby providing'an impermeable physical vicinity of the cathode.

2,939,323 Patented June 7,1960

barrier at the surface of the fused salt bath. The impermeable physical barrier extends somewhat above and below the surface of the fused salt bath between the anode and the cathode and thus completely separates the surface portion of the bath adjacent the anode from the surface portion of the bath adjacent the cathode. It is important that this impermeable barrier be electrically isolated from both the anode and the cathode. When thus isolated, the barrier, by a mechanism not presently understood, prevents the formation of the aforementioned surface film or skin of metallic titanium with the result ing improvement in the over-all electrolytic efficiency of the cell.

The process for electrorefining impure titanium metal to which my invention relates is fundamentally different from electrowinning processes for the production of titanium metal from titanium-containing compounds. In conventional electrowinning processes a titaniferous compound such as titanium tetrachloride or an alkali metal titanium double fluoride is employed as a source of the titanium metal that is electrodeposited at the cell cathode. Such electrowinning processes employ fused halide salt baths as the electrolyte and are conducted under electrolytic conditions that result in the evolution of a halogen gas such as chlorine at the cell anode. Because of the tice to provide an internal barrier in the cell between the anode and the cathode that will prevent the halogen evolved at the anode from entering the electrolyte in the The aforementioned barrier must efiect-ively prevent the evolved halogen from becoming dispersed throughout the bath but at the same time must permit an electrolyzing current to flow between the anode and the cathode; a conventional form of bar- ,rier comprises a wall of porous refractory material such evolved at the anode and no internal barrier is necessary for the successful operation of the process. Accordingly,

it is conventional practice to carry out the electrorefining of impure titanium in a cell that has no such internal barrier between the anode and the cathode so that the internal resistance of the cell will not be unnecessarily increased. The impermeable barrier employed at the surface of the fused salt melt in accordance with the present invention does not affect the aforementioned conventional practice in electrorefining processes with respect to the absence of internal barriers between the cell anode and cell cathode.

My invention will be better understood. from the following description taken in conjunction with the accompanying drawings of which Fig. 1 is a section through an electrorefining cell embodying my invention, and

Fig. 2 is; a sectional view of a modified form of th electrorefining cell shown in Fig. 1.

The electrorefining cell comprises a container 11 formed of non-corrosive impervious material such as stainless steel, nickel or nickel alloy fitted with a cover 12. The cell is provided with a gas inlet: 13 and a gas outlet 14 for establishing and maintaining an inert atmospheretherein. 'Ihe fused salt electrolyte 15 comprises at least one alkali metal halide or alkaline earth metal halide, and preferably a eutectic mixture of these halides cell.

as hereinafter more fully explained, together with a significant amount of titanium dichloride. 'Ihe impure titanium metal is employed as the anode of the cell or is electrically connected to the anode of the cell. Accordingly, as shown in Fig. 1, the impure titanium 17, which ordinarily is in the form of impure titanium sponge or' crystals or small pieces of titanium scram metal,-is placed in a previous container or basket 18 formed for example of a corrosion-resistant nickel alloy or graphite that is immune to' attack by the fused salt bath, and this container 18 is then immersed in the fused salt bath where it is electrically connected to the cell circuit as the anode thereof. Alternatively, as shown in Fig. 2, the impure titanium 17 is placed in the bottom of the container 11 of the electrolytic cell, and the entire container is employed as the anode. The cathode 19 on which the purified titanium metal deposits is formed of a material immune to attack by the fused salt bath, advantageously steel, nickel or corrosion-resistant nickel alloys.

"In accordance with my invention, an impermeable physical barrier 20 is disposed at the surface of the fused salt bath between the anode and the cathode so that the surface portion of the bath adjacent the anode is completely separated from the surface portion of the bath adjacent the cathode. The barrier extends only a short distance below the surface of the fused salt electrolyte so that the passage of the electrolyzing current through the electrolyte between the anode and the cahode is unimpeded and so that the presence of the barrier 20 in the cell does notsignificantly increase the internal resistance of the cell. The impermeable physical barrier 20 must, of course, be formed of a corrosion-resistant material immune to attack by the fused salt bath and must be physically spaced and electrically isolated from both the cell anode and the cell cathode (as, for example, by means of insulator 21). I have successfully used for this impermeable barrier a cylinder of a corrosion-resistant nickel alloy disposed so that it entirely surrounds the cell cathode at the surface of the fused salt bath in the However, other corrosion-resistant materials are effective as the barrier. Because the barrier is electrically isolated from the anode and cathode, both electrically conductive and non-conductive materials, of either metal- 'lic or non-metallic nature, are effective.

my invention comprise any one or more of the halides of the alkali metals and alkaline earth metals. Thus, the

chlorides, bromides, iodides and fluorides of sodium, potassium and lithium as well as the same halides of calcium, magnesium, barium and strontium may be used with advantage. Although an individual halide may be used as a single constitutent bath, I presently prefer to use a combination of these halides inasmuch as such combinations are characterized by relatively lower melting points than the individual salts. It is particularly advantageous, when using a combination of the aforementioned halides, to mix these halides in proportions approximating a eutectic composition in order to obtain baths with low melting points. For example, I have used with particularly satisfactory results a eutectic mixture composed of 5 mol percent of sodium chloride, 40 mol percent of potassium chloride and mol percent of lithum chloride, the resulting mixture having a reported melting point of 372 C. but actually melting at a temperature of about 345 C. Other useful eutectic mixtures are represented by a mixture composed of 48.5 mol percent of sodium chloride and 51.5 mol percent of calcium chloride having a anhydrous as possible and should be compounded of salts of high purity.

In addition to the aforementioned alkali and alkaline earth metal halides the bath must also contain a significant amount of titanium ions having a valence of less than four. These lower valence titanium ions may be supplied in the form of titanium trichloride or titanium dichloride, which are established in the molten salt bath by any one of a number of procedures known to the art. For example, titanium dichloride or trichloride from an extraneous source may be introduced directly into the bath; On the other hand, the titanium lower chloride may be formed in situ in the bath by dispersing'finely divided metallic titanium throughout the bath and by then bubbling titanium tetrachloride into the bath so that as a result of the reaction between the-metallic titanium and the titanium tetrachloride, titanium dichloride and some titanium trichloride are formed in the bath. The lower valence titanium chloride content of'the bath may also be established by continuously exposing the bath to a titanium tetrachloride atmosphere while maintaining an impressed cell voltage either below or above the decomposition voltage of one or more components of the carrier salt bath but sufficient to effect reduction of the titanium tetrachloride to a lower valence titanium chloride. Regardless of the source of the titanium lower chloride, its presence in the molten halide salt bath is essential in an amount suflicient to carry out successfully the electrorefin-ing process to which my invention relates. A fraction of a percent of lower titanium chloride is suflicient for this purpose, but economical operation generally requires 2% or more of the lower chloride.

The electrolysis is conducted at a cell voltage below that at which the bath decomposes with evolution of free chlorine or other halogen gas. The decomposition voltage of the fused halide bath will, of course, depend upon the composition and the temperature of the bath; in general, the higher the temperature of the bath, the lower is the decomposition voltage of a particular constituent of the bath. I have found that when a fused salt bath such as that described herein is maintained at a temperature of between 50 and 350 C. above its melting point, a cell voltage of between one volt and 3 volts is advantageously employed, the minimum voltage being that necessary to effect the refining of impure titanium and the maximum voltage being slightly below that at which the bath is decomposed with concomitant evolution of free halide at the anode. Y

The following example is illustrative of the practice of my invention:

Fine titanium particles containing 12% 0 were electrorefined in a cell arranged as in Figure 2 except that no barrier at the melt surface was used. The melt bath was the LiCl-KCl--NaCl eutectic described earlier, with the addition of 4% titanium in solution With an average valence of 2.23. At 550 C. a current of 15 amp. at 0.45 volt was passed for twelve hours between the cathode and the container serving as anode. A cathode deposit of 73 g. of coarsely-crystalline titanium metal was recovered, corresponding to a current efficiency of 50%.

Pieces of thin titanium metal foil were found clinging to the deposit at the level of the melt surface.

The cell was equipped with a 3%" high x 5 diamete cylindrical nickel alloy barrier surronding the 1" diameter cathode, the barrier protruding out of the melt for approximately half its height, as shown in Figure 2. In an experiment otherwise identical with the first, g. of titanium of quality equal to that of the first experiment were recovered, corresponding to a current efliciency of 70%. There was no evidence of the formation of titanium metal foil at the surface of the melt. t

I claim: I

1. In the method of electrore'fining impure metallic titanium by passing an electrolyzing current between the impure titanium at the anode and an inert cathode both in contact with a fused halide salt bath containing a lower valent titanium halide, the cell voltage being below that at which there is evolution of free halogen gas at the anode, substantially pure titanium being deposited at the cathode and tending to form a skin of the metal on the surface of the bath, the improvement which comprises substantially preventing the formation of titanium metal on the surface of the bath by positioning an impermeable physical barrier at the surface of the fused salt bath so as to extend above and below the surface of the fused salt bath between the anode and the cathode and thus completely separating the surface portion of the bath adjacent the anode from the surface portion of the bath adjacent the cathode, the barrier being electrically isolated from the anode and cathode and extending only a short distance below the surface of the bath so as not to impede the passage of the electrolyzing current between the anode and cathode.

2. In the method of electrorefining impure metallic titanium by passing an electrolyzing current between the *in contact with a fused halide salt bath containing a lower valent titanium halide, the cell voltage being below that at which there is evolution of free halogen gas at the anode, substantially pure titanium being deposited at the cathode and tending to form a skin of the metal on the surface of the bath, the improvement which comprises substantially preventing the formation of titanium metal on the surface of the bath by positioning an impermeable physical barrier at the surface of the fused salt bath so as to extend above and below the surface of the fused salt bath between the anode and the cathode and thus completely separating the surface portion of the bath adjacent the anode from the surface portion of the bath adjacent the cathode, the barrier being electrically isolated from the anode and cathode and extending only a short distance below the surface of the bath so as not to impede the passage of the electrolyzing current between the anode and cathode, the passage of the electrolyzing current through the fused salt bath between the anode and the cathode being unimpeded by said barrier.

3. In the method of electrorefining impure metallic titanium by passing an electrolyzing current between the impure titanium at the anode and an inert cathode both in contact with a fused halide salt bath containing a lower valent titanium halide, the cell voltage being below that at which there is evolution of free halogen gas at the anode, substantially pure titanium being deposited at the cathode and tending to form a skin of the metal on the surface of the bath, the improvement which comprises substantially preventing the formation of titanium metal on the surface of the bath by positioning a cylindrically shaped impermeable physical barrier at the surface of the fused salt bath, the cylindrically shaped barrier completely encircling the cathode at the surface of the fused salt bath and extending a distance above and below the surface of the bath and thus completely separating the surface portion of the bath adjacent the anode from the surface portion of the bath adjacent the cathode, the barrier being physically spaced and electrically isolated from the anode and cathode and extending only a short distance below the surface of the bath so as not to impede the 6 passage of the electrolyzing current between the anode and cathode.

4. In the method of electrorefining impure metallic titanium by passing an electrolyzing current between the impure titanium at the anode and an inert cathode both in contact with a fused halide salt bath containing a lower valent titanium halide, the cell voltage being below that at which there is evolution of free halogen gas at the anode, substantially pure titanium being deposited at the cathode and tending to form a skin of the metal on the surface of the bath, the improvement which comprises substantially preventing the formation of titanium metal on the surface of the bath by positioning an impermeable physical barrier formed of a corrosion-resistant metal at the surface of the fused salt bath so as to extend above and below the surface of the fused salt bath between the anode and the cathode and thus completely separating the surface portion of the bath adjacent the anode from the surface portion of the bath adjacent the cathode, the barrier being electrically isolated from the anode and cathode and extending only a short distance below the surface of the bath so as not to impede the passage of the electrolyzing current between the anode and cathode.

5. In the method of electrorefining impure metallic titanium by passing an electrolyzing current between the impure titanium at the anode and a corrosion-resistant metal cathode both in contact with a fused halide salt bath containing at least 2% by weight of a lower valent titanium halide, the cell voltage being within the range of about 1 to 3 volts, and substantially pure titanium being deposited at the cathode and tending to form a skin of the metal on the surface of the bath, the improvement which comprises substantially preventing the formation of titanium metal on the surface of the bath by positioning a cylindrically-shaped impermeable physical barrier of a corrosion-resistant metal at the surface of the fused salt bath, the cylindrically-shaped barrier completely encircling the cathode and extending a distance above and below the surface of the fused salt bath between the anode and the cathode and thus completely separating the surface portion of the bath adjacent the anode from the surface portion of the bath adjacent the cathode, the barrier being physically spaced and electrically isolated from the anode and cathode and extending only a short distance below the surface of the bath so as not to impede the passage of the electrolyzing current between the anode and cathode, the passage of the electrolyzing current through the fused salt bath between the anode and the cathode being unimpeded by said barrier.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE METHOD OF ELECTROEFINING IMPURE METALLIC TITANIUM BY PASSING AN ELECTROLYZING CURRENT BETWEEN THE IMPURE TITANIUM AT THE ANODE AND AN INERT CATHODE BOTH IN CONTACT WITH A FUSED HALIDE SALT BATH CONTAINING A LOWER VALENT TITANIUM HALIDE, THE CELL VOLTAGE BEING BELOW THAT AT WHICH THERE IS EVOLUTION OF FREE HALOGENN GAS AT THE ANODE, SUBSTANTIALLY PURE TITANIUM BEING DEPOSITED AT THE CATHOD AND TENDING TO FORM A SKIN OF THE METAL ON THE SURFACE OF THE BATH, THE IMPROVEMENT WHICH COMPRISES SUBSTANTIALLY PREVENTING THE FORMATION OF TILTANIUM METAL ON THE SURFACE OF THE BATH BY POSITIONING AN IMPERMEABLE PHYSICAL BARRIER AT THE SURFACE OF THE FUSED SALT BATH SO AS TO EXTEND ABOVE AND BELOW THE SURFACE OF THE FUSED SALT BATH BETWEEN THE ANODE AND THE CODE AND THUS COMPLETELY SEPARATING THE SURFACE PORTION OF THE BATH ADJACENT THE ANODE FROM THE SURFACE PORTION OF THE BATH ADJACENT THE CATHODE, THE BARRIER BEING ELECTRICALLY ISOLATED FROM THE ANODE AND CATHODE AND EXTENDING ONLY A SHORT DISTANCE BELOW THE SURFACE OF THE BATH SO AS NOT TO IMPEDE THE PASSAGE OF THE ELECTROLYZING CURRENT BETWEEN THE ANODE AND CATHODE. 